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The NFL has warned players that eating too much meat produced in Mexico or China could result in positive drug tests.

According to sources, the NFL Players Association proved to the NFL that the burgers and steaks Brown ate in Mexico caused the positive test. Consequently, on Tuesday, the drug program's independent administrator sent a letter warning players that consumption of too much meat in Mexico and China could cause a positive test for the anabolic substance clenbuterol.

Clenbuterol has been used to control asthma and as an unregulated weight-loss supplement. It is not used in any medication approved by the Food and Drug Administration and is banned by the NFL.

NFLPA associate general counsel Heather McPhee researched the hormone and discovered its connection to certain types of meat.

The letter advised players Tuesday to be cautious about their meals when traveling to Mexico or China and reminded them that the league's substance-abuse policy says "players are responsible for what is in their bodies."

It's a warning Brown likely understands all too well. He is tested many times during the season as a result of a 2010 performance-enhancing drug suspension. One such test came in November, after Brown and his wife had taken a bye week trip to Baja, California. A source said Brown had about 10 burgers and two steaks during the trip. The receipts from Brown's meals became evidence in his appeal.

After a months-long process, Brown was finally cleared in April, and his case offers a warning for other players.

The Texans will travel to Mexico to play the Oakland Raiders on Nov. 21, but Brown isn't likely to take a leisure trip to Mexico any time soon. Sources say that if he does, he'll stick to chicken and fish.